Aluminum alloys are used in the construction of heat exchangers, such as evaporators, radiators, heaters and condensers and the like, due to their light weight and excellent heat transfer properties. Heat exchangers are typically manufactured from aluminum alloy parts that are formed from rolled sheet or extruded products. The parts are typically assembled, fixtured, cleaned and joined in a brazing process. In a brazing process, two or more parts, each clad with an aluminum brazing alloy (e.g., an aluminum-silicon alloy), are positioned so that surfaces to be joined on the parts are in close proximity to each other. The parts are heated to a temperature which melts the braze alloy but not the core alloy on underlying parts. The braze alloy from each part melts together to close the gap which separates the parts. Upon cooling, the brazing alloy solidifies and forms a metallurgical bond between the parts. The brazing alloy is typically introduced onto the surfaces of the aluminum stock by cladding thereto in a roll bonding operation.
A common brazing practice includes cleaning of the formed parts using a suitable solvent to remove oils and the like from the surfaces to be brazed followed by application of a suitable flux to the pre-brazed parts to be joined. The fluxed parts are heated in a controlled atmosphere, such as dry nitrogen, to retard oxidation. Flux is used to reduce the oxides on the faying surfaces of the parts that are to be joined by brazing.
In most commercial brazing operations, flux is applied after fabrication of the individual parts to be brazed, either after assembly and fixturing of the parts (e.g., as a radiator, condenser, or heater) or prior to assembly for heat exchangers having internal brazed joints (e.g., evaporators) and prior to brazing. The flux may be applied directly as a dry powder or mixed with a carrier such as water or alcohol and applied as slurry over the entire work piece. When applied as slurry, the carrier is subsequently removed by a drying step, leaving the flux as a powder on the surface of the parts to be brazed.
Flux is only required in areas where metallurgical bonds or joints are required. Nevertheless, it is common manufacturing practice to apply flux over the entire assembly, often including the fixtures used to contain the parts in the furnace during brazing. This results in overuse and waste of flux, the need to clean the fixtures and increased maintenance of the furnace due to the corrosive nature of the flux. Moreover, the process of applying and removing excess flux is time consuming and expensive. It should be noted that flux is often loosely adhered to the parts as a powder. Hence, care must be taken to avoid removal of the flux during any handling of the fluxed parts prior to brazing.
An alternative to fluxing an entire assembly of parts is to apply flux to the metal sheets prior to working or forming the sheets into parts. Applying flux to a metal sheet prior to forming the sheet into a desired part is advantageous in that the flux can be applied only on the braze alloy cladding where joints are to be formed between parts. The unclad areas of the metal, which are not to be joined, can remain free of flux. However, flux coated brazing sheet has not found broad commercial applications due to the rigorous demands on the flux coating. The flux on prefluxed sheet must survive stamping and forming operations, not degrade when exposed to the forming lubricants and not interfere with the brazing operation.
Thus, a need exists for a brazing flux mixture which can be applied over an entire aluminum alloy brazing sheet or can be applied on the brazing sheet only where metallurgical bonds or joints are required, that is sufficiently durable to withstand processing operations such as stamping, forming, and handling prior to brazing and also provides good metallurgical bonds upon brazing.
It is therefore an object of this invention to provide a brazing flux mixture which can be applied over an entire aluminum alloy brazing sheet, or can be applied on the brazing alloy only where metallurgical bonds or joints are required, that is sufficiently durable to withstand processing operations and also provides good metallurgical bonds upon brazing.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description of some particularly preferred embodiments.